Two Rutan designs fathered the famous White Knight launch aircraft: the Proteus and the little-known Grasshopper...

The Custom Launch Aircraft included by Burt Rutan in a presentation was likely very similar to the Grasshopper...

... or maybe more like this one, from another presentation?

Customer:  NASA Dryden

Type:  large launch aircraft for liquid-powered manned booster system

Program:  Custom Launch Aircraft (CLA)

Powerplant: not defined

Significant date: 1992

Circa 1992, a team led by Antonio Elias (designer of the Pegasus booster) set up an informal "brain session" at NASA Dryden with several prominent aerospace engineers. Max Faget, C.C. Johnson and Burt Rutan were invited to reflect on this question: "Where to go with air-launch?" After putting their brains together, the team concluded to the following: 1°) The next size up would be similar to an Atlas II or Delta II in size and payload; 2°) It would be a two- stage LOX-kerosene (shades of Atlas and Delta) and 3°) It would require a custom-build odd-looking aircraft with a twin tail and a four-point undercarriage. The latter was driven by the desire of fuel, main LG, engines and payload all to cluster around the aircraft's CG.

The team then proceeded to sketch a very large, liquid-powered air-launched manned booster system. Faget and Johnson designed the spacecraft, a small, two-seat winged thing code-named Maliboo. Rutan and Elias worked more specifically on the launch aircraft, code-named Grasshopper, which was sort of a Proteus-style carrier aircraft. Rutan wanted to put rocket engines on the carrier aircraft to increase its flight path angle and avoid a wing in the rocket, while Elias wanted a wing on the rocket and no rocket engines on the aircraft. Elias says he still owns Burt's sketches from that session. Some ten years later, Rutan unveiled a (roughly) 1/3-scale version of Grasshopper called White Knight...

Further reference was made to a Rutan-designed Custom Launch Aircraft (CLA) and a picture appeared in a Burt Rutan presentation. Considering the description that was given by Elias and the picture of the CLA, it is safe to assume that the latter was either the same as the former, or perhaps a refined, formalized draft of the former. What is certain is that the Grasshopper concept was the direct forerunner, not only of White Knight and White Knight Two, but also of the Stratolaunch Roc, an early design of which can be seen below.

Population: not built

Specifications: not defined

Crew/passengers: not defined

Main sources:

The team that brainstormed on the air-launch system (left to right): Joe Allen, DWT (?), Max Faget, Antonio Elias, Burt Rutan, Bob Lovell and CC. Johnson.